FIGS. 3(a) and 3(b) show cross-sections of structures of the resonator end portion and the resonator direction of a prior art semiconductor laser. In FIGS. 3(a) and 3(b), the reference numeral 21 designates a substrate comprising p type GaAs, numeral 22 designates a current blocking layer comprising n type GaAs, numeral 23 designates a lower cladding layer comprising p type Al.sub.y Ga.sub.1-y As, numeral 24 designates an active layer comprising Al.sub.x Ga.sub.1-x As, numeral 25 designates an upper cladding layer comprising n type Al.sub.y Ga.sub.1-y As, numeral 26 designates a contact layer comprising n type GaAs, numeral 27 designates a stripe groove, and numerals 28 and 29 designate p and n electrodes comprising metal layers, respectively. Herein, the composition proportion x and y are chosen such that x&lt;y. Reference numerals 30 and 31 designate resonator end surfaces.
After growing an n type GaAs current blocking layer 22 on a p type GaAs substrate 21, a stripe groove 27 of a depth reaching the substrate 21 is produced. Subsequently, a lower cladding layer 23 comprising p type Al.sub.y Ga.sub.1-y As, an active layer comprising Al.sub.x Ga.sub.1-x As, an upper cladding layer comprising n type Al.sub.y Ga.sub.1-y As, and a contact layer 26 comprising n type GaAs are successively grown thereon, and resonator end surfaces 30 and 31 are produced by cleavages. Electrodes 28 and 29 are deposited to complete a laser shown in FIGS. 3(a) and 3(b).
When a forward voltage is applied between the p type electrode 28 and the n type electrode 29, a forward direction current larger than a threshold value flows through the active layer 24. The current is concentrated in the groove width of the active layer 24 due to the internal current confinement structure comprising the current blocking layer 22 and the groove 27. In addition to the double hetero junction (DH) in the thickness direction of the active layer 24, an effective refractive index difference is provided between groove 27 and its sides in the transverse direction. These structures effectively confine current carriers and light waves the active region. Thus, this semiconductor laser shows a high operating efficiency at a high temperature, at a low threshold current, and in a transverse fundamental mode.
This prior art semiconductor laser has no problems when it is used at a relatively low output level of about 5 mW. However, several problems arise when it is operated at a light output larger than about 20 to 30 mW.
Generally, the active layer 24 has a uniform thickness between the resonator end surface portions 30 and 31 as shown in FIG. 3(b), and it has uniform crystalline composition. In an AlGaAs series laser having such an active layer 24, the resonator end surfaces 30 and 31 become light absorption regions due to the lack of carriers because of a rapid surface recombination. When the light output is increased, the light absorption at the resonator end surfaces 30 and 31 is increased, and a cycle of light absorption, heat generation, temperature rising of the resonator end surfaces occurs. The resonator end surfaces are melted and destroyed at above a particular light density (several MW/cm.sup.2 in AlGaAs series). This phenomenon is called COD (Catastrophic Optical Damage). When COD arises, characteristics of the semiconductor laser is deteriorate and the laser device fails. Several attempts have been made to prevent COD. One way of preventing COD is to selectively diffuse impurities, such as p type impurities, into the active layer in the bulk of the resonator and n type impurities into the active layer at the interface between the active layer and the resonator end surfaces. Another way of preventing COD is selectively growing of a layer having a large energy band gap, relative to that of the resonator, at the resonator end surfaces.
These methods are intended to produce a non-absoring mirror (NAM) structure in which the energy band gap of the active layer at the resonator end surfaces is larger than that of the active layer in the bulk of the resonator and absorption at the resonator, end surfaces is reduced. When this NAM structure is thus adopted, it is possible to raise the light density at which a COD occurs more than one order of magnitude (several 10 MW/cm.sup.2) over that of a usual laser, and to obtain higher power output operation.
The prior art NAM structure has several disadvantages. A complicated production process and a high precision control technique for controlling the width and depth of diffusion and the position of the cleavage are required. The threshold current is also increased and the astigmatism is greatly increased in a case where a refractive index guide is not provided at the resonator end surfaces in the transverse direction.